Optical head with defocusing correction and spherical aberration correction

ABSTRACT

An optical head with excellent optical characteristics even when using a light source that substantially has a wavelength broadening is provided. The optical head includes a light source, an objective lens for focusing a light beam emitted from the light source on an information recording medium and a photodetector for detecting the light beam reflected from the information recording medium, wherein defocusing correction means and spherical aberration correction means are provided in an optical path between the light source and the information recording medium. Since both the defocusing correction means and the spherical aberration correction means are provided, while defocusing of a focused spot on the information recording medium caused by a wavelength broadening of the light source and chromatic aberration of the optical system can be corrected, the spherical aberration of the optical system caused by wavelength difference between a design wavelength and an incident wavelength also can be corrected. As a result, excellent optical characteristics can be obtained.

FIELD OF THE INVENTION

The present invention relates to an optical head used for an opticalrecording/reproduction device. In particular, the present inventionrelates to an optical head with excellent optical characteristics evenwhen using a light source that substantially has a wavelengthbroadening.

BACKGROUND OF THE INVENTION

As an important component part for reading out signals from opticalrecording media such as optical discs, e.g. compact discs (CD), DVDetc., optical card memories or the like, an optical head is available.In order to take out signals from the optical recording media, theoptical head needs to be equipped not only with a signal detectionfunction but also with a control mechanism such as a focusing servo, atracking servo and the like.

The optical head usually includes various optical components such as alight source, a photodetector, an objective lens, a focus/tracking errorsignal detecting element, a mirror, a collimator lens etc. A laser beamemitted from a light source is focused on an optical disc by anobjective lens (collective lens). The laser beam focused on the opticaldisc is reflected and detected by a photodetector. In this way, areproduction signal is read out. Moreover, focusing and tracking arecontrolled by a focus/tracking error signal detecting element, so thatsignals can be read out stably.

However, in the case of a light source that has a wavelength broadeningsuch as a semiconductor laser source to which high frequency currentsuperposition is applied, due to chromatic aberration (in particular,mainly due to color dispersion of a glass material) of the opticalsystem including the objective lens, defocusing of a focused spot on theinformation recording medium was likely to occur according to the degreeof wavelength broadening.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the conventionalproblem described above by providing an optical head that is capable ofobtaining excellent optical characteristics according to itsconfiguration in which both defocusing correction means and sphericalaberration correction means and provided. Thus, while defocusing of afocused spot on an information recording medium caused by a wavelengthbroadening of a light source and chromatic aberration of an objectivelens can be corrected, spherical aberration of the optical system causedby wavelength difference between a design wavelength and an incidentwavelength also can be corrected, and furthermore, if necessary,spherical aberration caused by an error in a standard thickness of abase material of the information recording medium can be corrected.

In order to achieve the above object, the optical head of the presentinvention includes a light source, an objective lens for focusing alight beam emitted from the light source on an information recordingmedium and a photodetector for detecting the light beam reflected fromthe information recording medium, wherein defocusing correction meansand spherical aberration correction means are provided in an opticalpath between the light source and the information recording medium. Inthe optical head described above, according to the configuration inwhich both the defocusing correction means and the spherical aberrationcorrection means are provided, while defocusing of a focused spot on theinformation recording medium caused by a wavelength broadening of thelight source and chromatic aberration of the objective lens can becorrected, spherical aberration of the optical system caused bywavelength difference between a design wavelength and an incidentwavelength also can be corrected, and furthermore, if necessary,spherical aberration caused by an error in a standard thickness of abase material of the information recording medium can be corrected. As aresult, excellent optical characteristics can be obtained.

In the optical head described above, the light source preferably has awavelength broadening in which a full width at half maximum of awavelength is substantially 0.3 nm or more. When the light source has awavelength broadening of such a range, defocusing is likely to occur onthe information recording medium, thereby degrading the focused spot.Therefore, the effect of the present invention is more significant.

Furthermore, it is preferable that the above wavelength λ satisfies arelationship of 0.3 μm≦λ≦0.5 μm. When the wavelength is within such arange, the occurrence of defocusing and the amount of sphericalaberration caused by the wavelength deviation is increased even more,and signals on the information recording medium may not be read outeasily. Therefore, the effect of the present invention is moresignificant.

Moreover, it is preferable that the defocusing correction meanssubstantially corrects defocusing of a focused spot on the informationrecording medium caused by a wavelength broadening of the light sourceand chromatic aberration of the objective lens. According to the opticalhead described above, defocusing caused by the objective lens, which isthe main factor for the defocusing of the focused spot, can becorrected, so that excellent optical characteristics can be obtained.

Furthermore, it is preferable that the optical head of the presentinvention further includes a collimator lens positioned between thelight source and the objective lens in an optical path for substantiallycollimating the light beam emitted from the light source, and that thedefocusing correction means substantially corrects defocusing of afocused spot on the information recording medium caused by a wavelengthbroadening of the light source and chromatic aberration of an opticalsystem including the objective lens and the collimator lens. Accordingto the optical head described above, defocusing of the focused spot canbe corrected more surely.

Moreover, it is preferable that a lens group including the defocusingcorrection means and the collimator lens substantially collimates thelight beam emitted from the light source and substantially corrects thedefocusing. According to the optical head described above, the lensgroup can substantially correct the chromatic aberration not only of theobjective lens but also of the entire optical system including thecollimator lens. Therefore, it is unnecessary to provide separatecorrection means of chromatic aberration for the objective lens. As aresult, the configuration of the objective lens can be simplified, andan optical head that is thinner in the height direction can be obtained.

Furthermore, the defocusing correction means preferably is a diffractiveoptical element. According to the optical head described above, thechromatic aberration can be eliminated by combining the diffractiveoptical element and the objective lens, and thus, a thinner and lighteroptical head can be obtained, compared to the configuration ofeliminating the chromatic aberration by using only refractive opticalelements.

Furthermore, the diffractive optical element preferably is a convexdiffractive optical lens. According to the optical head described above,the chromatic aberration can be eliminated by combining the convexdiffractive optical lens and the objective lens, that is, by combininglenses that are both convex lenses. Therefore, the numerical aperture ofthe collective lens for focusing the beam on the information recordingmedium can be shared between the objective lens and the defocusingcorrection element. Thus, the numerical aperture of the objective lenscan be suppressed.

Moreover, the defocusing correction means preferably is an afocaloptical element.

Furthermore, it is preferable that the afocal optical element hasstaircase steps with a step size L1, and that the step size L1substantially satisfies L1=m λ/(n−1), where λ is a wavelength, n is arefractive index of a material of the optical element and m is anarbitrary integer.

Moreover, the afocal optical element preferably is a combined lens ofconvex and concave types. According to the optical head described above,the optical loss can be reduced.

Furthermore, the defocusing correction means preferably is a combinedlens of concave and convex types that are both refractive opticalelements.

Moreover, it is preferable that the spherical aberration correctionmeans substantially corrects the spherical aberration of the objectivelens and the defocusing correction means caused by wavelength differencebetween a design wavelength and an incident wavelength. According to theoptical head described above, the spherical aberration caused by theobjective lens, which is the main factor of the spherical aberrationcaused by the wavelength deviation, can be corrected, so that excellentoptical characteristics can be obtained.

Furthermore, it is preferable that the optical head of the presentinvention further includes a collimator lens positioned between thelight source and the objective lens in an optical path for substantiallycollimating the light beam emitted from the light source, and that thespherical aberration correction means substantially corrects thespherical aberration of the optical system including the objective lensand the collimator lens and the defocusing correction means caused bywavelength difference between a design wavelength and an incidentwavelength. According to the optical head described above, the sphericalaberration caused by the wavelength deviation can be corrected moresurely.

Furthermore, it is preferable that the spherical aberration correctionmeans substantially corrects the spherical aberration caused by an errorin a standard thickness of a base material of the information recordingmedium. According to the optical head described above, the sphericalaberration caused by an error in a standard thickness of a base materialof the information recording medium can be corrected in addition to thecorrection of the spherical aberration caused by the wavelengthdeviation. As a result, even more excellent optical characteristics canbe obtained.

Moreover, the spherical aberration correction means preferably is madeof a liquid crystal element in which a refractive index distribution ischangeable. According to the optical head described above, the phasedistribution can be determined according to the wavelength change, sothat the spherical aberration can be corrected excellently.

Moreover, the liquid crystal element preferably has 4 or more segmentedareas electrically divided in the radial direction. According to theoptical head described above, by changing the amount of electricpotential to be applied to each of the segmented portions according tothe wavelength change, the spherical aberration can be corrected. Also,by providing 4 or more segmented areas, the value of the sphericalaberration can be reduced to half or less.

Furthermore, it is preferable that a numerical aperture of a collectivelens for focusing a light beam on the information recording medium isshared between the objective lens and the defocusing correction means.According to the optical head described above, the numerical aperture ofthe objective lens can be suppressed, so that the objective lens can bemanufactured easily.

Furthermore, it is preferable that the defocusing correction means is aconvex diffractive optical lens, and that a numerical aperture of theobjective lens is in the range between 8 times to 20 times higher than anumerical aperture of the diffractive optical lens.

Furthermore, the objective lens preferably includes 2 or more lenses.According to the optical head described above, the numerical aperturecan be increased easily.

Moreover, a numerical aperture of the objective lens preferably is 0.55or higher.

Furthermore, it is preferable that the objective lens includes 2 or morelenses, and that the numerical aperture thereof is 0.65 or higher. Asthe numerical aperture is increased, defocusing is more likely to occur,so that the effect of defocusing correction is more significant.

Furthermore, it is preferable that the defocusing correction means andthe spherical aberration correction means are integrated. According tothe optical head described above, the defocusing correction means andthe spherical aberration correction means can be formed as onecomponent. As a result, a thinner device can be manufactured, andposition adjustment is no longer necessary.

Moreover, it is preferable that at least one selected from thedefocusing correction means and the spherical aberration correctionmeans is driven integrally with the objective lens by an actuator.According to the optical head described above, the optical axis of theobjective lens matches at least one of the optical axes of thedefocusing correction means and the spherical correction means in thedriven state, so that excellent optical characteristics can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing the basic structure of an optical headaccording to Embodiment 1 of the present invention and how a light beamis propagated.

FIG. 2 is a graph showing the relationship between a refractive indexand a wavelength of a glass material for forming an objective lens etc.of an optical head according to Embodiment 1 of the present invention.

FIG. 3( a) is a cross-sectional view of a defocusing correction elementin an optical head according to Embodiment 1 of the present invention.

FIG. 3( b) is a plan view of a defocusing correction element in anoptical head according to Embodiment 1 of the present invention.

FIG. 4 is a plan view of a spherical aberration correction element of anoptical head according to Embodiment 1 of the present invention.

FIG. 5( a) is a phase distribution chart of a spherical aberrationcorrection element at the time when an incident wavelength is a designwavelength in an optical head according to Embodiment 1 of the presentinvention.

FIG. 5( b) is a phase distribution chart of a spherical aberrationcorrection element at the time when an incident wavelength is increasedfrom a design wavelength in an optical head according to Embodiment 1 ofthe present invention.

FIG. 5( c) is a phase distribution chart of a spherical aberrationcorrection element at the time when an incident wavelength is reducedfrom a design wavelength in an optical head according to Embodiment 1 ofthe present invention.

FIG. 6 is a side view showing the basic structure of an optical headaccording to Embodiment 2 of the present invention and how a light beamis propagated.

FIG. 7 is a side view showing the basic structure of an optical headaccording to Embodiment 3 of the present invention and how a light beamis propagated.

FIG. 8( a) is a cross-sectional view of an objective lens with adefocusing correction element formed in an optical head according toEmbodiment 3 of the present invention.

FIG. 8( b) is a plan view of an objective lens with a defocusingcorrection element formed in an optical head according to Embodiment 3of the present invention.

FIG. 9 is a side view showing the basic structure of an optical headaccording to Embodiment 4 of the present invention and how a light beamis propagated.

FIG. 10 is a side view showing the basic structure of an optical head inanother example of Embodiment 4 of the present invention and how a lightbeam is propagated.

FIG. 11 is a side view showing the basic structure of an optical head inanother example of Embodiment 4 of the present invention and how a lightbeam is propagated.

FIG. 12 is a side view showing the basic structure of an optical head inanother example of Embodiment 4 of the present invention and how a lightbeam is propagated.

FIG. 13 is a side view showing the basic structure of an optical headaccording to Embodiment 5 of the present invention and how a light beamis propagated.

FIG. 14 is a side view showing the basic structure of an optical head inanother example of Embodiment 5 of the present invention and how a lightbeam is propagated.

FIG. 15 is a side view showing the basic structure of an optical headaccording to Embodiment 6 of the present invention and how a light beamis propagated.

FIG. 16 is a side view showing the basic structure of an optical headaccording to Embodiment 6 of the present invention and how a light beamis propagated.

FIG. 17 is a side view showing the basic structure of an optical headaccording to Embodiment 7 of the present invention and how a light beamis propagated.

FIG. 18 is a side view showing the basic structure of an optical head inanother example of Embodiment 7 of the present invention and how a lightbeam is propagated.

DETAILED DESCRIPTION OF THE INVENTION EMBODIMENT 1

First, an optical head according to Embodiment 1 of the presentinvention will be explained in detail by using FIG. 1 to FIG. 5 and bydetermining the coordinate axis as illustrated in each drawing.

FIG. 1 is a side view showing the basic structure of the optical headaccording to Embodiment 1 of the present invention and how a light beamis propagated.

As illustrated in FIG. 1, on an optical path from a light source 1 to aninformation recording medium 11, which is an optical disc such as DVD,CD etc., an objective lens 4, a defocusing correction element 5 asdefocusing correction means, a spherical aberration correction element 9as spherical aberration correction means, a mirror 12, a focus/trackingerror signal detecting element 8 and a collimator lens 3 are arranged.

The light source 1 is a light source having a substantially wavelengthbroadening. This light source 1 is, for example, a light source with awavelength of 405 nm, which is integrated together with a photodetector(not shown) inside a light source/photodetector unit 10. In the presentembodiment, as the light source 1, a semiconductor laser to which highfrequency current superposition is applied is used.

An emitted beam 2, which is a laser beam emitted in the y-axis directionfrom a semiconductor laser, i.e. the light source 1, is changed to asubstantially parallel beam 6 of, for example, 4 mm in beam diameter bythe collimator lens 3 and is transmitted through a diffraction-typefocus/tracking error signal detecting element 8 (using the zero-orderdiffracted light), and the optical path is deflected in the z-axisdirection by the mirror 12. Then, the laser beam 6 deflected in thez-axis direction passes through the spherical aberration correctionelement 9 and the defocusing correction element 5 and is focused (aconvergent beam 7) on the information recording medium 11 by theobjective lens 4.

The laser beam 7 reflected by the information recording medium 11 isdeflected in the opposite direction and passes through the objectivelens 4, the defocusing correction element 5, and the sphericalaberration correction element 9 and the mirror 12 in order. Then, thelaser beam is divided by the diffraction-type focus/tracking errorsignal detecting element 8 (using the first-order diffracted light) andfocused on the photodetector by the collimator lens 3 for detectingsignals.

FIG. 2 is a graph showing the relationship between a refractive indexand a wavelength of a glass material for forming the objective lens etc.in the optical head of the present embodiment. In the presentembodiment, for reducing noise caused by the return beam, high frequencysuperposition is applied to the light source 1 of the semiconductorlaser, and as a result, a full width at half maximum of the wavelengthis broadened. It is clear from FIG. 2 that, with regard to the glassmaterial for forming the objective lens 4 and the collimator lens 3, therefractive index thereof is dependent on the wavelength. Therefore, whenthe light source 1 has a wavelength broadening, because of thiswavelength broadening and chromatic aberration of the lenses, a focaldistance of the lens corresponding to each wavelength is changed. Inother words, considering the entire wavelength of the light source 1,when the wavelength differs, the focal point is also shifted. As aresult, there was a phenomenon that defocusing was likely to occur inthe focused spot on the information recording medium 11. In addition,FIG. 2 shows an example in which VC79 was used as the glass material,but the same change can be observed by using other glass materials.

For prevention of the defocusing described above, as illustrated in FIG.1, the present invention uses the defocusing correction element 5, whichis shown in detail in FIG. 3. FIG. 3( b) is a plan view of thedefocusing correction element 5, and FIG. 3( a) is a cross-sectionalview taken on line I—I of FIG. 3( b). The defocusing correction element5 is, as illustrated in FIG. 3, a convex diffractive optical lens havinga saw-tooth shaped portion 14 formed on a glass substrate 5 a.

This defocusing correction element 5 is used in combination with theobjective lens 4 and is configured to eliminate the chromatic aberration(achromatism) of the objective lens 4. In other words, the defocusingcorrection element 5 has a correction ability of such a degree that thedefocusing of the focused spot on the information recording medium 11caused by the wavelength broadening of the light source 1 and thechromatic aberration of the objective lens 4 can be correctedsubstantially. In addition, “to correct substantially” does not onlymean “to correct completely”, but the meaning of “to correct to such adegree that there is no problem in practical use” is included herein(the same applies to the following).

One example of the defocusing correction element 5 is configured to havea diameter d of 4 mm, a numerical aperture (NA) of about 0.05, a groovedepth L of 0.8 μm and a period T that is reduced gradually towards theperiphery (for example, the utmost peripheral period of 8 μm). When thegroove depth L is determined to satisfy L=2λ/(n−1) substantially, wherea central wavelength of the light source 1 is λ and a refractive indexof the material of the defocusing correction element 5 is n, highdiffraction efficiency can be obtained.

As described above, in the present embodiment, a diffractive opticalelement is used as the defocusing correction element 5, and a convexdiffractive optical lens is used as this diffractive optical element. Inother words, by combining the convex diffractive optical lens and theconvex objective lens 4, achromatism can be achieved. In order toachieve such achromatism only by refractive optical elements, a concavelens can be used in combination with a convex lens.

According to the present embodiment, compared to the configurationachieving achromatism only by refractive optical elements, a thinner andlighter optical head can be obtained.

Furthermore, since achromatism can be achieved by combining thedefocusing correction element 5 and the objective lens 4 in this way,that is, by combining both convex lenses, the NA of the collective lensfor focusing the beam on the information recording medium 11 can beshared between the objective lens 4 and the defocusing correctionelement 5.

For example, in the present embodiment, the NA of the objective lens 4is determined as 0.6, and the NA of the defocusing correction element 5is determined as 0.05, so that the NA is 0.65 in total. As the NA of theobjective lens 4 is increased, it is more difficult to mold the lens,but by sharing the NA in this way, the objective lens 4 can bemanufactured more easily. In addition, although it differs according tothe glass material used for the objective lens 4, in the case where theachromatic conditions are satisfied, the NA of the objective lens 4 isin the range of about 8 times to 20 times higher than the NA of thediffractive optical lens 5.

Furthermore, other than the objective lens 4, the collimator lens 3 etc.also are used as the optical system, so that the defocusing correctionelement 5 more preferably is configured to correct not only thechromatic aberration of the objective lens 4 but also to substantiallycorrect the chromatic aberration of the entire optical system includingthe collimator lens 3 and so forth. This configuration can be realizedby providing the defocusing correction element 5 with a correctionability of such a degree that the chromatic aberration of the entireoptical system can be corrected substantially.

Moreover, in the case of using the defocusing correction means asdescribed above, the present inventors found out that, when thewavelength of the light source was changed, for example, by about ±10 nmaccording to an environmental temperature, spherical aberration was morelikely to occur.

The reason for this phenomenon was examined intensively, and as aresult, it became clear that, also with regard to the occurrence of suchspherical aberration, the main factor is the fact that the refractiveindex of the glass material is dependent on the wavelength asillustrated in FIG. 2. Moreover, it became clear that, in the case ofusing the defocusing correction element 5 as illustrated in FIG. 1, whenthe wavelength of the light source 1 is changed, the value of thespherical aberration is increased by about 1.2 to 1.7 times higher thanthe value obtained in the case where the defocusing correction element 5is not used.

For prevention of such spherical aberration, the spherical aberrationcorrection element 9 is used in the present embodiment as illustrated inFIG. 1, and the details thereof are shown in FIG. 4. FIG. 4 is a planview showing one example of a spherical aberration correction element.The spherical aberration correction element 9 shown in FIG. 4 is made ofa liquid crystal element in which a refractive index distribution ischangeable, and the liquid crystal is held between glass plates.

More specifically, a glass substrate 9 a has a plurality of transparentsegmented electrodes 13 a to 13 i arranged in concentric circles, and 4or more segmented areas electrically divided in the radial direction (9divisions in the example shown in FIG. 4) are provided. When an electricpotential is applied to each of the transparent segmented electrodes 13a to 13 i, a refractive index of the liquid crystal is changed accordingto the amount of electric potential, which is realized as a phasedistribution.

FIG. 5 shows phase distribution charts of the spherical aberrationcorrection element 9 at the time when an incident wavelength waschanged. When the wavelength of the light source 1 is a designwavelength of, for example, λ=405 nm, spherical aberration due towavelength deviation does not occur, so that the phase amount is 0throughout the entire area as shown in FIG. 5( a). In this case, it isfine that there is no difference in the electrical potential between therespective transparent segmented electrodes 13 a to 13 i.

However, when the wavelength was increased by 10 nm to λ=415 nm, theamount of spherical aberration was changed from 0, for example, to −45 mλ. Therefore, a different electric potential was applied to each of thetransparent segmented electrodes 13 a to 13 i so that the phasedistribution formed two crests with, for example, the maximum values ofaround 0.5 to 1 rad as illustrated in FIG. 5( b). In this way, theamount of spherical aberration was reduced. In this case, the amount ofchange in the spherical aberration amount was calculated by thedetection mechanism of the spherical aberration amount. When the amountof spherical aberration changes from zero, the amount of change iscalculated by the detection mechanism, and a phase distribution isformed according to this amount of change.

Next, when the wavelength was reduced by 10 nm to λ=395 nm, the amountof spherical aberration was changed, for example, to 46 m λ, Therefore,a different electric potential was applied to each of the transparentsegmented electrodes 13 a to 13 i so that the phase distribution formedtwo troughs with, for example, the minimum values of around 0.5 to 1 radas illustrated in FIG. 5( c). In this way, the amount of sphericalaberration was reduced as in the case where the wavelength wasincreased. As described above, when the wavelength was changed and thespherical aberration occurred, by changing the amount of electricpotential to be applied to each of the transparent segmented electrodes13 a to 13 i in accordance with the wavelength change, the sphericalaberration could be corrected substantially almost to a degree ofexcellence.

The phase distributions illustrated in FIGS. 5B, 5C show ideal cases. Inthe present embodiment, the transparent segmented electrodes 13 a to 13i are used, so that the phase distributions of FIGS. 5B, 5C are realizedby approximating stepwise. In order to improve the degree ofapproximation, it is preferable to increase the divided number of thetransparent segmented electrodes and to finely control an electricpotential to be applied in accordance therewith. However, it becameclear that, by electrically dividing it into 4 or more segments(four-segmented electrodes) in the radial direction, the value of thespherical aberration can be reduced to half or less.

As described above, according to the present embodiment, both thedefocusing correction means 5 and the spherical aberration correctionmeans 9 are provided. Therefore, while the defocusing of a focused spoton the information recording medium 11 caused by the wavelengthbroadening of the light source 1 and the chromatic aberration of theoptical system can be corrected substantially, the spherical aberrationof the optical system caused by the wavelength difference between thedesign wavelength and the incident wavelength also can be correctedsubstantially. As a result, excellent optical characteristics can beobtained.

In particular, in the case where the wavelength broadening is such thatthe full width at half maximum of the wavelength is substantially 0.3 nmor larger, it is likely that defocusing occurs, thereby degrading thefocused spot. Therefore, the effect of this aspect of the presentinvention is more significant. Furthermore, as illustrated in FIG. 2,when the wavelength λ is within the range of 0.3 μm≦λ≦0.5 μm, therefractive index change is larger. In other words, within such a range,the refractive index is changed greatly due to a slight change of thewavelength, compared to the range of λ>0.5 μm. Thus, it became clearthat, within such a range, the occurrence of defocusing and the amountof spherical aberration caused by the wavelength deviation wereincreased even more, and that signals on the information recordingmedium 11 may not be read out easily. Therefore, the effect of thepresent invention is more significant when the wavelength λ is in therange of 0.3 μm≦λ≦0.5 μm. This wavelength range is rounded off to onedecimal places, so that 0.25 μm and 0.54 μm also are included withinthis wavelength range.

Furthermore, as the NA of the collective lens (in this embodiment, thecombined lens of the objective lens 4 and the defocusing correctionelement 5) for focusing the beam on the information recording medium 11is increased, defocusing is more likely to occur. Therefore, the effectof the present invention in this case also was more significant.Particularly, it was effective when the NA was 0.55 or higher.

In addition, elements other than the objective lens 4, the collimatorlens 3 etc. also are used as the optical system, so that the sphericalaberration correction element 9 more preferably is configured to correctnot only the spherical aberration of the objective lens 4 but also tosubstantially correct the spherical aberration caused by thedisplacement between the design wavelength and the incident wavelengthof the entire optical system including the collimator lens 3 and soforth. This configuration can be realized by determining the phasedistribution of the spherical aberration correction element 9 such thatthe spherical aberration of the entire optical system can be correctedsubstantially.

Moreover, in the present embodiment, the defocusing correction element5, the spherical aberration correction element 9 and the objective lens4 were driven integrally by an actuator. According to thisconfiguration, the three elements can be driven while the optical axesthereof are matched, so that excellent optical characteristics can beobtained. In addition, by driving either the defocusing correctionelement 5 or the spherical aberration correction element 9 with theobjective lens 4 integrally with an actuator, such effect can beobtained to some degree.

Furthermore, the phase distribution of the spherical aberrationcorrection means 9 can be determined such that not only the sphericalaberration caused by the wavelength deviation of the light source itselfis corrected, but also the spherical aberration caused by amanufacturing error of a base material thickness of the informationrecording medium 11 (for example, a manufacturing error in a designvalue of 0.6 mm) also is corrected substantially. By forming such aphase distribution, the optical characteristics can be improved evenmore.

Furthermore, in the present embodiment, the spherical aberrationcorrection element 9 and the defocusing correction element 5 arearranged as illustrated in FIG. 1, but the arrangement is not limitedhereto. Both elements may be positioned in the optical path between theinformation recording medium 11 and the light source 1. Moreover, byusing the objective lens 4 in the arrangement of a finite system, thecollimator lens 3 may be omitted.

Furthermore, the defocusing correction element 5 made of a diffractiveoptical element may be positioned to face the opposite direction of thedirection described in this embodiment. In addition, the collimator lens3 and the focus/tracking error signal detecting element 8 may bepositioned in the reverse order.

EMBODIMENT 2

Next, an optical head in Embodiment 2 of the present invention will beexplained with reference to FIG. 6 by mainly referring to the point thatis different from Embodiment 1 described above. FIG. 6 is a side viewshowing the basic structure of the optical head according to Embodiment2 of the present invention and how a light beam is propagated.

According to the configuration of the optical head in the presentembodiment, the defocusing correction element 5 a, which is defocusingcorrection means, and the spherical aberration correction element 9 a,which is spherical aberration correction means, are integrated. Byforming the defocusing correction element 5 a on top of the sphericalaberration correction element 9 a, the defocusing correction means andthe spherical aberration correction means can be formed as onecomponent. As a result, a thinner device can be manufactured, andposition adjustment is no longer necessary.

Furthermore, the defocusing correction element 5 a and the sphericalaberration correction element 9 a may be positioned in the reverse ofthe case illustrated in FIG. 6.

EMBODIMENT 3

Next, an optical head according to Embodiment 3 will be explained withreference to FIGS. 7, 8 by mainly referring to the point that isdifferent from Embodiment 1 described above. FIG. 7 is a side viewshowing the basic structure of the optical head according to Embodiment3 and how a light beam is propagated. FIG. 8( b) is a plan view of anobjective lens with a defocusing correction element formed according toEmbodiment 3, and FIG. 8( a) is a cross-sectional view taken on lineII—II of FIG. 8( b).

As illustrated in FIG. 8, a concentric circular pattern is formed on anobjective lens 4 a. This pattern has, as illustrated in FIG. 8( a), across-section substantially of a saw-tooth shape having a step s, and adefocusing correction element 5 b is formed by this saw-tooth shape. Byintegrating the defocusing correction element 5 b with the objectivelens 4 a, the optical head can be miniaturized, and position adjustmentalso can be simplified.

Here, the step s is formed to satisfy s=2λ/(n−1) substantially, where λis a central wavelength of the light source 1 and n is a refractiveindex of a material for forming the defocusing correction element 5 b.When a design wavelength λ, i. e. a single wavelength λ without anywavelength broadening, enters, a phase difference of a light beamcorresponding to the step s substantially becomes 2π (there issubstantially phase matching), so that the optical loss hardly occurs asin the case where the step s does not exist (as in the case where thedefocusing correction element 5 b does not exist), and the light beam isfocused excellently by the objective lens 4 a.

On the other hand, when an incident wavelength is shifted from thedesign value, a phase difference in a groove depth of the defocusingcorrection element 5 b is shifted from 2π. Therefore, the defocusingcorrection element 5 b performs a wavefront conversion so that a focalfluctuation of the objective lens 4 a (chromatic aberration) which iscaused by the wavelength deviation can be extinguished mutually.

In other words, as the wavelength of the parallel beam 6 is elongated,the refractive index of the glass material of the objective lens 4 a isreduced, so that the focal distance of the objective lens 4 a iselongated. However, as for the defocusing correction element 5 b, thephase difference at the step is reduced from 2π, and thus, the emittedbeam of the defocusing correction element 5 b will be a convergent beam,which practically acts to reduce the focal distance of the objectivelens 4 a, and as a whole, the fluctuation of the focal distance isextinguished.

In addition, in the present embodiment, for substantially achieving thephase matching at the step of the defocusing correction element 5 b, thestep s was determined such that the phase difference was 2π, but thephase difference may be determined to be substantially an integralmultiple of 2π. In this case, however, the optical loss is larger thanin the former case.

EMBODIMENT 4

Next, an optical head according to Embodiment 4 of the present inventionwill be explained with reference to FIG. 9 by mainly referring to thepoint that is different from Embodiment 1 described above. FIG. 9 is aside view showing the basic structure of the optical head according toEmbodiment 4 of the present invention and how a light beam ispropagated.

In this embodiment, as illustrated in FIG. 9, the objective lensincludes two lenses 4 b, 4 c, and together with the defocusingcorrection element 5, a total of three lenses form a collective lens,for example, with a high NA of NA0.85. Moreover, the base materialthickness (thickness of a protective layer) of the information recordingmedium 11 a was 0.6 mm in the previous embodiments, but the thickness isreduced to 0.1 mm in the present embodiment.

In the present embodiment, the base material is thinner, and theabsolute value of the optical path length passing through the basematerial is smaller, so that the increase of coma aberration caused bythe inclination of the information recording medium 11 a can besuppressed, and the NA of the objective lens can be increased.

In this embodiment, as in Embodiment 1, both the defocusing correctionmeans 5 and the spherical aberration correction means 9 are provided.Therefore, while the defocusing of a focused spot on the informationrecording medium 11 a caused by the wavelength broadening of the lightsource and the chromatic aberration of the optical system can becorrected, the spherical aberration of the optical system caused by thewavelength difference between the design wavelength and the incidentwavelength also can be corrected substantially.

In this case, when the NA of the objective lens was to be 0.65 orhigher, excellent characteristics could not be obtained with theconfiguration of one objective lens as in Embodiment 1, so that it wasnecessary to include 2 or more lenses as in this embodiment. Also, asexplained in Embodiment 1, defocusing was likely to occur as the NA ofthe collective lens for focusing the light beam on the informationrecording medium was higher. The present invention is effectiveparticularly when the NA of the collective lens is 0.55 or higher. Thetest results showed that, in the case where the NA of the combinedcollective lens was 0.65 or higher as in the present embodiment,defocusing was more likely to occur. Therefore, the effect of defocusingcorrection is even more significant when 2 or more objective lenses arecombined and the NA of the collective lens is determined to be 0.65 orhigher, as in the present embodiment.

Furthermore, when the wavelength is in the range of 0.3 μm≦λ≦0.5 μm, theoccurrence of defocusing is increased even more, and the effect of thedefocusing correction is significant as in Embodiment 1.

FIG. 10 is a side view showing the basic structure of an optical head inanother example of the present embodiment and how a light beam ispropagated. In the embodiment shown in FIG. 9, for preventing defocusingfrom occurring, the defocusing correction element 5, which is a convexdiffractive optical lens having a saw-tooth shaped portion, is used. Theembodiment shown in FIG. 10 is configured to eliminate chromaticaberration by combining a concave lens 5 e and a convex lens 4 b thatare both refractive optical elements.

The configuration of the embodiment illustrated in FIG. 11 is the samewith that shown in FIG. 6 except for the configuration of theinformation recording medium and that of the objective lens. In otherwords, in the present embodiment, a lens 4 c is provided in addition tothe objective lens 4 bto form a collective lens with a high NA.Furthermore, as with the embodiment shown in FIG. 6, the defocusingcorrection element 5 a is integrated with the spherical aberrationcorrection element 9 a, so that the optical head can be miniaturized,and position adjustment also can be simplified.

The configuration of the embodiment illustrated in FIG. 12 is the samewith that shown in FIG. 7 except for the configuration of theinformation recording medium and that of the objective lens. In otherwords, in the present embodiment, a lens 4 c is provided in addition tothe objective lens 4 ato form a collective lens with a high NA.Furthermore, as with the embodiment shown in FIG. 7, the defocusingcorrection element 5 b is integrated with the objective lens 4 a, sothat the optical head can be miniaturized, and position adjustment alsocan be simplified.

EMBODIMENT 5

Next, an optical head according to Embodiment 5 will be explained withreference to FIG. 13 by mainly referring to the point that is differentfrom Embodiment 1 described above. FIG. 13 is a side view showing thebasic structure of the optical head according to Embodiment 5 of thepresent invention and how a light beam is propagated.

In the present embodiment, as a defocusing correction element 5 c, anafocal optical element having staircase steps with a step size L1 isused, and the step size L1 substantially satisfies L1=m λ/(n−1), where λis a wavelength, n is a refractive index of a material of theaforementioned optical element and m is an arbitrary integer.

The defocusing correction element 5 c has a typical pattern of adiffraction-type lens in which the period is reduced gradually towardsthe periphery. As illustrated in FIG. 13, the cross-section of thedefocusing correction element 5 c has a staircase shape, and with regardto the step size L1 of this staircase shape, when a design wavelengthenters, a phase difference of the light against the step L1substantially becomes an integral multiple of 2π. Therefore, in case ofthe design wavelength, the state is equivalent to that without anyelement (this is the reason for calling it an afocal optical element),and the substantially parallel beam 6 with the design wavelength passesthrough as it is.

However, in the case where the wavelength is changed from the designwavelength, the phase difference is shifted. Therefore, as inEmbodiments 3 and 4, the parallel beam is changed to either a divergentbeam or a convergent beam, which serves to extinguish the focalfluctuation generated in the objective lens 4.

FIG. 14 is a side view showing the basic structure of an optical head inanother example of the present embodiment and how a light beam ispropagated. The configuration of the embodiment illustrated in FIG. 14is the same as that shown in FIG. 13 except for the configuration of theinformation recording medium and that of the objective lens. In otherwords, in the present embodiment, the objective lens includes two lenses4 b, 4 c to form a collective lens with a high NA. Moreover, as with theembodiment shown in FIG. 13, the defocusing correction lens 5 c isprovided, and when the wavelength is changed from the design wavelength,the focal fluctuation generated in the objective lens can beextinguished.

In addition, the defocusing correction element 5 c may be positioned, asillustrated in FIGS. 13, 14, in the optical path between the informationrecording medium 11 and the mirror 12, but it is also possible toposition the defocusing correction element 5 c in the optical pathbetween the light source 1 and the mirror 12.

EMBODIMENT 6

Next, an optical head according to Embodiment 6 will be explained withreference to FIG. 15 by mainly referring to the point that is differentfrom Embodiment 1 described above. FIG. 15 is a side view showing thebasic structure of the optical head according to Embodiment 6 and how alight beam is propagated.

In the present embodiment, as a defocusing correction element 5 d, anafocal optical element is used, in which a convex lens and a concavelens made of glass materials with different Abbe numbers are combined(however, a refractive index of the glass material is substantially thesame when an incident wavelength is a design wavelength). The principleof this element is the same as the afocal optical element explained inEmbodiment 5. By using the combined lens of convex and concave typesthat are both refractive optical elements, the effect of a smalleroptical loss can be obtained.

FIG. 16 is a side view showing the basic structure of an optical head inanother example of the present embodiment and how a light beam ispropagated. The configuration of the embodiment illustrated in FIG. 16is the same as that shown in FIG. 15 except for the configuration of theinformation recording medium and that of the objective lens. In otherwords, in the present embodiment, the objective lens includes two lenses4 b, 4 c to form a collective lens with a high NA. Moreover, as with theembodiment shown in FIG. 15, the defocusing correction element 5 dformed by the combined lens of convex and concave types is provided, andthe effect of a smaller optical loss can be obtained.

The defocusing correction element 5 d is positioned in the optical pathbetween the light source 1 and the mirror 12 in the examples of FIGS.15, 16, but the defocusing correction element 5 d may be positioned inthe optical path between the information recording medium 11 and themirror 12. When the defocusing correction element 5 d is positioned inthe optical path between the light source 1 and the mirror 12, theeffect of reducing the height to form a thinner optical head can beobtained.

EMBODIMENT 7

Next, an optical head according to Embodiment 7 will be explained withreference to FIGS. 17, 18 by mainly referring to the point that isdifferent from Embodiment 1 described above. FIG. 17 is a side viewshowing the basic structure of the optical head according to Embodiment7 of the present invention and how a light beam is propagated.

In the example shown in FIG. 17, in the optical path between thefocus/tracking error signal detecting element 8 and the light source 1,a lens 3 a and a combined lens of a concave lens 15 a and a convex lens15 b are arranged in series to form a lens group 16. The lens group 16not only serves to correct chromatic aberration but also acts as acollimator lens.

According to the present embodiment, the lens group 16 can substantiallycorrect the chromatic aberration not only of the objective lenses 4 b, 4c but also of the entire optical system including the collimator lens 3.Therefore, it is unnecessary to provide separate correction means ofchromatic aberration for the objective lenses 4 b, 4 c. As a result, theconfiguration of the objective lens can be simplified, and an opticalhead that is thinner in the height direction can be obtained.

FIG. 18 is a side view showing the basic structure of an optical head inanother example of the present embodiment and how a light beam ispropagated. In the example shown in FIG. 18, in the optical path betweenthe focus/tracking error signal detecting element 8 and the light source1, the lens 3 and the defocusing correction element 5 are arranged inseries to form a lens group 17. Also in the example shown in FIG. 18, asin the example shown in FIG. 17, the lens group 17 not only serves tocorrect chromatic aberration but also acts as a collimator lens.

According to the present embodiment, the lens group 17 can substantiallycorrect the chromatic aberration not only of the objective lenses 4 b, 4c but also of the entire optical system including the collimator lens 3.Therefore, it is unnecessary to provide separate correction means ofchromatic aberration for the objective lenses 4 b, 4 c. As a result, theconfiguration of the objective lens can be simplified, and an opticalhead that is thinner in the height direction can be obtained.

The optical heads of the present invention have been explained byreferring to Embodiments 1 to 7 above, but the present invention is notlimited to these embodiments. Optical heads, in which the configurationsof the optical heads in each of the embodiments are combined, also areincluded in the present invention, and the same effects as describedabove can be achieved.

In addition, the objective lens and the collimator lens used in theabove embodiments are named such for convenience, and they are the sameas those generally called lenses.

Furthermore, in the embodiments described above, the optical disc wasused as an example of the information recording medium, but the presentinvention also can be applied to card-shaped, drum-shaped, ortape-shaped products that are designed to reproduce media with aplurality of different specifications such as thickness, recordingdensity etc. by using the same kind of informationrecording/reproduction device.

As described above, excellent optical characteristics can be obtained inthe present invention according to the configuration in which both thedefocusing correction means and the spherical aberration correctionmeans are provided, and thus, while the defocusing of a focused spot onthe information recording medium caused by the wavelength broadening ofthe light source and the chromatic aberration of the optical system canbe corrected, the spherical aberration of the optical system caused bythe wavelength difference between the design wavelength and the incidentwavelength also can be corrected, and furthermore, if necessary, thespherical aberration caused by an error in the standard thickness of thebase material of the information recording medium can be corrected.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. An optical head comprising a light source, an objective lens forfocusing a light beam emitted from the light source on an informationrecording medium and a photodetector for detecting the light beamreflected from the information recording medium, wherein defocusingcorrection means for correcting a defocused state caused by a wavelengthchange and a wavelength broadening and spherical aberration correctionmeans as a separate element from the defocusing correction means areprovided in an optical path between the light source and the informationrecording medium, and the spherical aberration correction meanssubstantially corrects a spherical aberration occurring due to thedefocusing correction means in accordance with an amount of thespherical aberration.
 2. The optical head according to claim 1, whereinthe light source has a wavelength broadening in which a full width athalf maximum of a wavelength is substantially 0.3 nm or more.
 3. Theoptical head according to claim 2, wherein the wavelength λ of theemitted light beam satisfies a relationship of 0.3 μm≦λ≦0.5 μm.
 4. Theoptical head according to claim 1, wherein the defocusing correctionmeans substantially corrects defocusing of a focused spot on theinformation recording medium caused by chromatic aberration of theobjective lens.
 5. The optical head according to claim 1, furthercomprising a collimator lens positioned between the light source and theobjective lens in an optical path for substantially collimating thelight beam emitted from the light source, wherein the defocusingcorrection means substantially corrects defocusing of a focused spot onthe information recording medium caused by a wavelength broadening ofthe light source and chromatic aberration of an optical system includingthe objective lens and the collimator lens.
 6. The optical headaccording to claim 5, wherein a lens group comprising the defocusingcorrection means and the collimator lens substantially collimating thelight beam emitted from the light source and substantially correct thedefocusing.
 7. The optical head according to claim 1, wherein thedefocusing correction means is a diffractive optical element.
 8. Theoptical head according to claim 7, wherein the diffractive opticalelement is a convex diffractive optical lens.
 9. The optical headaccording to claim 1, wherein the defocusing correction means is anafocal optical element.
 10. The optical head according to claim 9,wherein the afacal optical element has staircase steps with a step sizeL1, and the step size L1 substantially satisfies L1 =mλ/(n−1t), where λis a wavelength, n is a refractive index of a material of the opticalclement and m is an arbitrary integer.
 11. The optical head according toclaim 9, wherein the afocal optical element is a combined lens of convexand concave types.
 12. The optical head according to claim 1, whereinthe defocusing correction means is a combined lens of concave and convextypes that are both refractive optical elements.
 13. The optical headaccording to claim 1, wherein the spherical aberration correction meanssubstantially corrects the spherical aberration of the objective lensand the defocusing correction means caused by wavelength differencebetween a design wavelength and an incident wavelength.
 14. The opticalbead according to claim 13, wherein the spherical aberration correctionmeans further substantially corrects spherical aberration caused by anerror in a standard thickness of a base material of the informationrecording medium.
 15. The optical head according to claim 1, furthercomprising a collimator lens positioned between the light source and theobjective lens in an optical path for substantially collimating thelight beam emitted from the light source, and wherein the sphericalaberration correction means substantially corrects spherical aberrationof the optical system including the objective lens and the collimatorlens and the defocusing correction means caused by wavelength differencebetween a design wavelength and an incident wavelength.
 16. The opticalhead according to claim 15, wherein the spherical aberration correctionmeans further substantially corrects spherical aberration caused by anerror in a standard thickness of a base material of the informationrecording medium.
 17. The optical head according to claim 1, wherein thespherical aberration correction means substantially corrects sphericalaberration caused by an error in a standard thickness of a base materialof the information recording medium.
 18. The optical head according toclaim 1, wherein the spherical aberration correction means is made of aliquid crystal element in which a refractive index distribution ischangeable.
 19. The optical head according to claim 18, wherein theliquid crystal element has 4 or more segmented areas electricallydivided in the radial direction.
 20. The optical head according to claim1, wherein a numerical aperture of a collective lens for focusing alight beam on the information recording medium is shared between theobjective lens and the defocusing correction means.
 21. The optical headaccording to claim 20, wherein the defocusing correction means is aconvex diffractive optical lens, and a numerical aperture of theobjective lens is in the range between 8 times to 20 times higher than anumerical aperture of the diffractive optical lens.
 22. The optical headaccording to claim 20, wherein the objective lens includes 2 or morelenses.
 23. The optical head according to claim 1, wherein a numericalaperture of the objective lens is 0.55 or higher.
 24. The optical headaccording to claim 1, wherein the objective lens includes 2 or morelenses, and the numerical aperture thereof is 0.65 or higher.
 25. Theoptical head according to claim 1, wherein the defocusing correctionmeans and the spherical aberration correction means are integrated. 26.The optical head according to claim 1, wherein at least one selectedfrom the defocusing correction means and the spherical aberrationcorrection means is driven integrally with the objective lens by anactuator.